In accordance with the fact that density of recording onto optical disc is caused to be high, skew margin of optical disc becomes less. For this reason, at the optical pick-up, skew sensor is used to detect inclination quantity of optical disc with respect to the optical axis of an optical pick-up (object lens) to control the optical pick-up by skew servo mechanism so that the optical axis of the object lens is perpendicular to the signal recording surface of the optical disc in accordance with this inclination quantity.
For example, as shown in FIG. 1 of the Japanese Patent Publication Laid Open No. 1997-161291, a conventional skew sensor comprises a light emitting element comprised of LED disposed at the body portion of the optical pick-up so that light is irradiated toward the signal recording surface of the optical disc, and bisected photo-diodes disposed at the body portion of the optical pick-up and serving to receive light reflected at the signal recording surface of the optical disc thus to take out a difference signal corresponding to inclination quantity of the optical recording medium from the both photo-diodes to detect this difference signal as a skew signal, and to control the skew servo mechanism by this skew signal so that the optical axis of the optical pick-up (optical axis of the object lens) is perpendicular to the signal recording surface of the optical recording medium.
In the conventional skew sensor as described above, since the light emitting element and the photo-diodes which are dedicated for skew detection are required, there is the problem that the number of parts is increased so that cost of the optical pick-up is increased.
Further, in the conventional skew sensor, since difference between output signals of bisected photo-diodes is taken out to thereby detect inclination quantity of the optical disc, when distance between the optical disc and the skew sensor changes, even if inclination quantity of the optical disc is the same, light quantity distribution of beams on the photo-diode for skew detection changes. As a result, error might take place in the inclination quantity of the optical disc.
In addition, since the distance between the optical disc and the skew sensor changes in dependency upon the kind of optical disc like CD or DVD, etc., the state of chucking with respect to turn table of the optical disc, and/or way of warping of the optical disc, etc., such distance changes at all times. For this reason, error is included at all times in an output of a light detection element under such circumstances so that precise skew detection becomes difficult.